1. Field of the Invention
Embodiments of the invention generally relate to a porous dielectric film for use in electronic devices. More particularly, the invention relates to porous dielectric films formed by removal of soluble porogens.
2. Description of the Related Art
The integrated circuit industry constantly reduces the separation between conductive layers of material in order to achieve smaller and faster integrated circuits. As feature sizes in the integrated circuits decrease, problems arise related to signal or resistor/capacitor (RC) delay, increased power consumption, and crosstalk or capacitive coupling between nearby conductors. The capacitive coupling increases noise along nearby conductors, which may be interpreted as a signal causing improper operation of the device that uses the integrated circuit. Decreasing the capacitance between the conductors provides one option for reducing the RC delay and the capacitive coupling. Since the capacitance between two conductors increases substantially in proportion to the dielectric constant (k) of the medium separating them, this can be achieved by using a dielectric film having a low K to separate the conductors.
Silicon dioxide (SiO2), which has a k of approximately 4.0, has long been used in integrated circuits as the primary insulating material. However, the k must be below 4.0 for optimum operation of current and future miniaturized high speed integrated circuits. Certain organic or polymeric materials provide a dielectric material with a lower k than SiO2. However, such materials tend to have limited thermal stability and mechanical strength. A number of carbon doped silicon oxide materials are currently available and provide low k films having a k of around 2.7 or less, such as Black Diamond™ dielectric layers developed by Applied Materials, Inc. Spin on glass (S.O.G.) and chemical vapor deposition (CVD) provide known methods for forming the SiO2 and carbon doped silicon oxide films. However, as the integrated circuits become smaller the dielectric films will require a k below 2.0. Fluoropolymers are known to have a k down to approximately 1.7. However, because of their poor thermal and mechanical properties fluoropolymers are not suitable for use in the manufacture of integrated circuits, where temperatures of 400° C. to 425° C. are normally encountered during subsequent processing steps.
The dielectric constant of dry air is about 1.0. Thus, one way to obtain a material with a low k is to use a porous low dielectric material in which a significant fraction of the bulk volume consists of space or air. In order to obtain a k below 2.0, the dielectric film may require more than 50% meso and nano pores. The effective k is determined by the combination of the k of the air or other gases filling the pores and the k of the dielectric material. Porous dielectric materials may be fabricated in many different structural forms with many different compositions. Therefore, such materials offer the possibility of achieving a low k and having composition and/or structural features resulting in acceptable mechanical, thermal, electrical and chemical properties. Generally, the films require a small pore size in order to maintain mechanical strength and make the film suitable for filling small patterned gaps.
In order to improve the mechanical strength and properties of porous dielectric films, various techniques have been developed. Post cures and coatings have been used to improve mechanical properties without significantly affecting the k of the dielectric layer. For example, exposing a porous dielectric film to an electron beam improves the mechanical strength by decreasing the hardness of the film and improving its modulus of elasticity.
Porous dielectric films have previously been fabricated by a number of different methods. Foams made by blowing air or other gases through a material to create voids or by liberating gas throughout the material provide pores too large to be used in integrated circuits with sub-micron characteristic feature sizes. Sol-gels that are typically based upon hydrolysis and condensation provide another class of porous material used for making a dielectric material. In operation the sol is a colloidal suspension of solid particles in a liquid that transforms into a gel due to growth and interconnection of the solid particles. Thereafter, a pore fluid is evaporated or dried from the gel to provide the porous film. Another alternative method for forming a porous material includes applying a composition having a decomposable polymer or volatile organic compound therein that can be later decomposed or evaporated and removed to form the porous dielectric layer. All of the prior methods for forming porous films have disadvantages that include requiring heating at various stages, precise control of reaction conditions, toxic solutions, and outgasing of unreactive precursors. Heating steps used in creating porous films with prior methods can alter the properties of the integrated circuit.
Therefore, there exists a need for a simple processing method that preferably utilizes non-toxic solutions to provide a porous dielectric material with controlled pore size.